Hard to choose, due to different writing styles, but it's a toss-up between Sir Terry Pratchett and Jim Butcher. David Weber is still in the running, however.
I gotta go with Sir Pterry on that one. Butcher isn't bad but he's got a few problems that keep me from considering him on Pratchett's level. He's too heavy-handed in places, rather formulaic in others, relies too heavily on Harry being a cosmic punching bag, and he's got too many internal inconsistancies.
The last point, especially, irks me since he contradicts himself constantly on the relative power and abilities of the "NPCs" from Christian mythology compared to those from older religions, like the Faeries (Eldest Gruff vs Magog comes to mind) or Shagnasty The Skinwalker.